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The scanner in my program is asking the user for second input, why is the scanner doing so?

I do not understand the reason why the method below is asking me for a second input when i type an integer as an input. Note, as far as i know, the method works fine when the input is a non integer, however when the user types an integer, then the method askes the user for a second input.
The method is:
public void setNumOfStudents() {
while (true) {
try {
System.out.print("Number of students: ");
String number = scanner.nextLine();
if (number.isEmpty()) {
while (number.isEmpty()) {
System.out.println("Please enter a valid number.");
System.out.print("Number of students: ");
number = scanner.nextLine();
}
}
numOfStudents = Integer.parseInt(number);
listOfStudents = new String[numOfStudents];
break;
}
catch (InputMismatchException | NumberFormatException e) {
System.out.println("Please enter an integer.");
continue;
}
}
}

FileWriter not receiving all input and mysterious dangling newline issue

I am trying to write a Java program in which the user specifies how many "student records" they would like to input, followed by the student's name, age, and GPA, which then gets stored as text. However, I am having a problem with my text not including all entered data and a mysterious dangling newline that I cannot get rid of.
Here is my program:
import java.io.*;
import java.util.Scanner;
public class CreateFile {
public static void main(String[] args) throws IOException {
Scanner input = new Scanner(System.in);
FileWriter fwriter = new FileWriter("c:\\Students.dat");
PrintWriter StudentFile = new PrintWriter(fwriter);
String name = " ";
String next = " ";
int age = 0;
int hm = 0;
double gpa = 0.0;
System.out.print("How many student records would you like to enter: ");
hm = input.nextInt();
for (int x = 1; x <= hm; x++) {
System.out.print("Enter Name: ");
name = input.nextLine();
input.nextLine();
System.out.print("Enter Age: ");
age = input.nextInt();
System.out.print("Enter GPA: ");
gpa = input.nextDouble();
next = input.nextLine();
StudentFile.println(name);
StudentFile.println(age);
StudentFile.println(gpa);
}
StudentFile.close();
System.exit(0);
}
}
Here is sample input and output to illustrate my issues:
run:
How many student records would you like to enter: 3
Enter Name: Jon
Enter Age: 20
Enter GPA: 3.4
Enter Name: Bill
Enter Age: 24
Enter GPA: 3.6
Enter Name: Ted
Enter Age: 34
Enter GPA: 3.9
This is the produced text file:
20
3.4
Bill
24
3.6
Ted
34
3.9
Why doesn't it store the first name entered? Why isn't there a newline in the first entry, but it is in the others?

The problem is that you're using nextLine() when you need to be using next(). I'm assuming you put the second input.nextLine() in there because you were initially having a problem where it would print out "Enter Name: " and then immediately "Enter Age: ". nextLine() is telling your program to skip whatever is there, and not to wait for it. The reason that this paradigm worked at all for any of your entries is that you put next = input.nextLine() at the bottom of your loop. Here's a fix:
package createfile;
import java.io.*;
import java.util.Scanner;
public class CreateFile {
public static void main(String[] args) throws IOException {
Scanner input = new Scanner(System.in);
FileWriter fwriter = new FileWriter("c:Students.dat");
PrintWriter StudentFile = new PrintWriter(fwriter);
String name = " ";
String next = " ";
int age = 0;
int hm = 0;
double gpa = 0.0;
System.out.print("How many student records would you like to enter: ");
hm = input.nextInt();
for (int x = 1; x <= hm; x++) {
System.out.print("Enter Name: ");
name = input.next();
System.out.print("Enter Age: ");
age = input.nextInt();
System.out.print("Enter GPA: ");
gpa = input.nextDouble();
StudentFile.println(name);
StudentFile.println(age);
StudentFile.println(gpa);
}
StudentFile.close();
System.exit(0);
}
}
You could also just move your input.nextLine() above name=input.nextLine() and it would have the same effect.

The other examples only work if you don't have names like "James Peter" - in their code examples only James would be saved as name.
I'd prefer this:
System.out.print("How many student records would you like to enter: ");
hm = input.nextInt();
input.nextLine();
for (int x = 1; x <= hm; x++) {
System.out.print("Enter Name: ");
name = input.nextLine();
System.out.print("Enter Age: ");
age = input.nextInt();
input.nextLine();
System.out.print("Enter GPA: ");
gpa = input.nextDouble();
input.nextLine();
StudentFile.println(name);
StudentFile.println(age);
StudentFile.println(gpa);
}

This is the corrected for loop:
for ( int x = 1; x <= hm; x++ )
{
System.out.print( "Enter Name: " );
name = input.next();
input.nextLine();
System.out.print( "Enter Age: " );
age = input.nextInt();
input.nextLine();
System.out.print( "Enter GPA: " );
gpa = input.nextDouble();
next = input.nextLine();
StudentFile.println( name );
StudentFile.println( age );
StudentFile.println( gpa );
}
Some things you may want to consider:
Handle the IOException - it should not be ignored!!
Use the methods hasNextXXX() of the Scanner to check if something is available.
Refactor your usage of the variable next, it's never really used.
It's not necessary to call System.exit( 0 ) from the main method - rather use the return statement with a meaningful value.

Java Do While Validation

How do I change this while(true) into a do while so when the user enters a number they will be given back details but if they enter * the system will close.
while (true){
System.out.print("Enter number: ");
int option = keyboard.nextInt();
out.writeInt(option);

char option;
do {
System.out.print("Enter number: ");
option = keyboard.nextChar();
out.writeChar(option);
} while (option != '*')

You may want to use nextLine() or next() to receive the input and parse them accordingly:
do{
System.out.print("Enter number: ");
String str = keyboard.nextLine();
int option = 0;
if(str.matches"[0-9]+"){
option = Integer.parseInt(str);
System.out.println(option);
}
else if(str.equals("*"))
System.exit(0); //or use break; if you want to exit the loop
}while(whatever); //whatever == true

If you change it to allow the user to input a String and then convert to int if possible, you can catch any errors and break out if the user enters a '*' character:
import java.util.*;
class MainInput{
public static void main(String[] args){
Scanner keyboard = new Scanner(System.in);
int option;
String input;
do{
System.out.print("Enter number: ");
input = keyboard.nextLine();
try{
option = Integer.parseInt(input);
System.out.println("You entered the value: " + option);
}
catch(Exception ex) {
if (!input.equals("*")){
System.err.println("Invalid input, please enter numbers only.");
}
}
}while(!input.equals("*"));
}
}
If you use an exit value like (-1), you can continue to process input with nextInt() and becomes even easier. You can do this with a simple do/while:
import java.util.*;
class MainInput{
public static void main(String[] args){
Scanner keyboard = new Scanner(System.in);
int option = 0;
do{
System.out.print("Enter number: ");
option = keyboard.nextInt();
System.out.println("You entered the value: " + option);
}while(option != -1);
}
}

do{
System.out.print("Enter number: ");
Scanner sc = new Scanner(System.in);
String str = sc.nextLine();
}while(!str.equals("*"))
edit: if you want to play with numbers as integer or double. Just let me know and I'll add casted version as your needs.
int number;
do{
System.out.print("Enter number: ");
Scanner sc = new Scanner(System.in);
String str = sc.nextLine();
number = Integer.parseInt(str);
}while(!str.equals("*"))
now you have a integer number.

what is the solution to StringIndexOutOfBoundsException

when i use s.charAt(0) while s is an string input from the user, I get this as an error even though the program runs the first half of the program.
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 0
at java.lang.String.charAt(String.java:658)
at Shopping.main(Shopping.java:22)
What's the solution to this program? here is my code.
import java.util.Scanner;
public class Shopping {
public static void main(String[] args){
Scanner keyboard = new Scanner(System.in);
System.out.println("Programmed by Raymond Lee");
System.out.println("Welcome to Shopper's Paradise");
ShoppingCart cart = new ShoppingCart();
System.out.print("Enter the name of the first item: ");
String item = keyboard.nextLine();
System.out.print("Enter the quantity: ");
int quantity = keyboard.nextInt();
System.out.print("Enter the price: ");
double price = keyboard.nextDouble();
cart.addToCart(item, price, quantity);
System.out.print("Enter the name of the next item or Q to quit: ");
String quit = keyboard.nextLine();
char choice = quit.charAt(0);
while((choice != 'Q' && choice != 'q') || quit.length() != 1) {
quit = item;
System.out.print("Enter the quantity: ");
quantity = keyboard.nextInt();
System.out.print("Enter the price: ");
price = keyboard.nextDouble();
cart.addToCart(item, price, quantity);
System.out.print("Enter the name of the next item or Q to quit: ");
quit = keyboard.nextLine();
choice = quit.charAt(0);
}
System.out.println(cart);
}
}

The error is occuring at this line
char choice = quit.charAt(0);
This is due to the fact that when you call
double price = keyboard.nextDouble();
then nextDouble leaves the newline in the input stream. So when following is called
String quit = keyboard.nextLine();
then result of nextLine is empty string, results in the given error when you try to use charAt method.
To resolve this error, simply change following
String quit = keyboard.nextLine();
To
String quit = keyboard.next();
Hope this helps

How to avoid stack build up?

I'm trying to figure out how I can write this method to avoid the stack buildup from recursively calling the method in the exception?
Here is the wording of my instructions:
Read a number, use an exception handler to make sure it is an int number and then add to the ArrayList object, aryList.
Here is my attempt:
public void createOriginalAryList() {
Scanner keyboard = new Scanner(System.in);
System.out.println("Enter a number: ");
try {
int number = keyboard.nextInt();
aryList.add(number);
while(keyboard.hasNextInt()) {
System.out.println("Enter a number: ");
number = keyboard.nextInt();
aryList.add(number);
}
} catch(InputMismatchException ime) {
System.out.println("Invalid number submitted! Try again.");
createOriginalAryList();
}
System.out.println(aryList);
}
Any suggestions are greatly appreciated!

Simply use a do-while loop:
Scanner keyboard = new Scanner(System.in);
boolean redo = false;
do {
System.out.println("Enter a number: ");
redo = false;
try {
int number = keyboard.nextInt();
aryList.add(number);
while(keyboard.hasNextInt()) {
System.out.println("Enter a number: ");
number = keyboard.nextInt();
aryList.add(number);
}
} catch(InputMismatchException ime) {
redo = true;
System.out.println("Invalid number submitted! Try again.");
}
}
while(redo);
System.out.println(aryList);
Since initializing the Scanner keyboard each time is useless, it is put before the loop.